The present invention relates to a transfer sheet used as a transfer material in a copying machine and a printer of an indirect dry electrophotographic system and to a color image forming process of forming a color toner image on the above-described transfer sheet and fixing the toner image by pressing and/or heating.
Hitherto, with the increase in the requirement of a color image formation and digital image formation by a copying machine and a printer of an electrophotographic system, improvement of the image quality of an electrophotographic system has been investigated. Particularly, in a full color copying machine or printer of an electrophotographic system, to obtain images of a high image quality, the employment of a digital system proceeds for the input and output of images, and an image-input method, a processing method of the input images, a development method, a transfer method, a fixing method, etc., have been largely improved. Also, the developer and the image-forming material for a photoreceptor have been improved in response to the digital high precision and high-coloring color recording. Furthermore, because color toners are required to have a good melting property and a good color mixing property at applying heat in a fixing apparatus, color toners having a low softening temperature are generally used.
On the other hand, in a full-color copying machine and printer of an electrophotographic system, transfer sheets of mainly a wood free paper base have been used but recently the improvement of the image quality by the increase of the resolution of a full-color copying machine and printer has made further progress and in order to obtain full-color images of a high image quality having high-grade feeling and a high coloring property and without having roughness, coated papers having a coating layer made of a pigment and a binder resin have been frequently used as the transfer sheets.
Furthermore, in a full-color copying machine or printer of an electrophotographic system, when a full-color image is formed by transferring toner images and fixing the transferred images using color toners having particle sizes of from several xcexcm to several tens xcexcm, in the case of using a wood free paper having many voids on the surface thereof as the transfer sheet, the protuberance of the toner is not so large but in the case of using a transfer sheet having less voids on the surface layer and having an air permeability of at least 100 seconds, the protuberance of the toner becomes large particularly at a high-density image portion and a continuous film is liable to from. Also, to improve the melting property and color mixing property of color toners, a binder resin, such as a polyester resin having a low molecular weight of from about 2,000 to 9,000 as the number average molecular weight Mn is mainly used for the color toners but the toner fixed on a coated paper on which a continuous film is liable to form, the behavior of the toner cannot overtake the expansion and contraction of the coated paper caused by the change in humidity and it sometimes happens that an imaged portion is crazed.
For solving these problems, Japanese Patent Laid-Open No. 19178/1994 proposes a method of preventing the occurrences of crazing on the coating layer of a coated paper and crazing of images formed thereon due to a long-term storage and impact by incorporating a styrene-butadiene copolymer in a transparent resin coating layer made of a styrene-acryl-based resin. However, when a paper is used as the base material of a transfer sheet, the behavior of a toner on the coating layer and at the imaged portion cannot overtake the elongation of the base material at high humidity and imaged portion is sometimes crazed.
The present invention has been made for solving the above-described problems and provides an electrophotographic transfer sheet using a paper-base coated paper mainly made of pulp fibers, which does not cause crazing at the imaged portions even under high humidity, shows high coloring of images, and does not give roughness.
An aspect of the present invention is an electrophotographic transfer sheet having a base material made of pulp fibers as a main body having formed on at least one surface thereof a coating layer. An elongation of the sheet in the cross direction when a humidity is changed from 25% RH to 90% RH at a temperature of 20xc2x0 C. is not higher than about 0.65% and a water content of the sheet at opening a package thereof as measured according to JIS P8127 is approximately in the range from 3.0 to 6.5%.
Another aspect of the present invention is an electrophotographic transfer sheet having a base material made of pulp fibers as a main body. At least about 30% by weight of the pulp fibers are a dry pulp, and a water content of the sheet at opening a package thereof as measured according to JIS P8127 is approximately in the range from 3.0 to 6.5%.
Another aspect of the present invention is also an electrophotographic transfer sheet having a base material made of pulp fibers as a main body having formed on at least one surface thereof a coating layer. The coating layer is a transparent resin layer made of a thermoplastic resin as a main constituent, a fiber orientation ratio of the base material is approximately in the range from 1.05 to 1.45, and a water content of the sheet at opening a package thereof as measured according to JIS P8127 is approximately in the range from 3.0 to 6.5%.
Another aspect of the present invention is a process of forming a color image by transferring a color toner image formed by an electrophotographic process onto an electrophotographic transfer sheet and fixing the transferred toner image by applying heat. The color toner contains a polyester binder resin having a number average molecular weight Mn approximately in the range from 1,000 to 9,000, and the electrophotographic transfer sheet is the transfer sheet described in the above aspects.